This invention relates to the manufacture of multi-chip modules and, in particular, to a method for making a multi-chip module by manufacturing a plurality of chips using a direct attach method and encapsulating the multi-chip module.
Semiconductor devices, colloquially referred to as chips, are used in the manufacture and operation of many electronic devices which have become an integral part of everyday life. Manufacturers of electronic devices are forced to make products having increased functionality and better quality. This often means having to put more chips in a given work area (i.e., increasing chip density).
One way of satisfying this need for increased chip density is to use multi-chip modules. Today's methods for making multi-chip modules, however, are often time-consuming and costly and result in low assembly yields. Two basic components employed in existing methods of multi-chip module manufacture are a printed circuit board and a lead frame. During the manufacturing process, the printed circuit board must be, in some fashion, conductively connected to the lead frame such that, when necessary, any component on the printed circuit board may have a conductive path on which to receive sufficient electrical voltage to operate properly.
Today, two well-known methods are used for effecting this conductive connection between a printed circuit board and a lead frame. As shown in FIG. 1A, the first of these involves the use of wire attachments 100 to create a connection between a printed circuit board 105 and a lead frame 110. In this method, a conventional printed circuit board and a conventional lead frame are used. During assembly, the printed circuit board 105 is fixedly mounted on top of the lead frame's central platform 115. Subsequently, in a similar manner to that used in the well-known wire bonding procedure for chip manufacture, a thin wire is first bonded to a printed circuit board bonding pad or hole and spanned to a lead frame finger. Next, the wire is bonded to the lead frame finger. Last, the wire is clipped and the entire process repeated at the next printed circuit board bonding pad or hole. Additionally, to provide a conductive pathway between a lead-frame finger and a particular component, a bonding wire extends from the particular component and the bonding pad. Again, this component/bonding pad connection is created using the well-known wire bonding procedure discussed above. While this may be a conceptually simple process, it is nonetheless critical because the wires must be accurately placed, every wire must establish good electrical contact at both ends, and the span between the printed circuit board's bonding pad or hole and the lead frame finger must have a kink-free arc positioned at a safe distance from surrounding wires. Wire bonding is done with either gold or aluminum wires. Both of these materials are highly conductive and sufficiently ductile to withstand deformation during the bonding process and yet remain strong and reliable.
Today, use of the most commonly used of these bonding materials is gold. Gold is the best-known room-temperature conductor, an excellent heat conductor, and resistant to oxidation and corrosion. Thus, gold can be melted to form a strong bond with the printed circuit board bonding pads or holes without oxidizing during the process.
There are a number of drawbacks to using bonding wires to attach a printed circuit board to a lead frame. For example, the high cost of gold makes its use as bonding wire material expensive. Bonding wire breakage during encapsulation results in reduced assembly yields, an undesirable condition to most manufacturers. Gold is susceptible to induction, and induction causes circuit noise. Circuit noise, in turn, may cause a module to operate improperly or unpredictably or to not operate at all. The use of bonding wires prohibits the mounting of chips on both sides of the printed circuit board thereby limiting the surface area available for chip placement. As the area available for chip placement on a given module decreases, so does the ability to meet customer-demanded increases in functionality without expanding work areas. The use of bonding wires requires extremely accurate wire placement; such accuracy demands time thereby slowing down the assembly process.
The second of the two well-known methods for conductively connecting a printed circuit board that provides conductive paths between lead frame fingers and components involves the use of a laminated printed circuit board 150 and is illustrated in FIG. 1B. A printed circuit board 155 is made using multiple layers that are laminated to form one structure. During the printed circuit board's lamination process, lead frame fingers are inserted into the printed circuit board 150 such that a portion of each lead frame finger is fixed within the layers of laminate. The other portion of each lead frame finger protrudes beyond the perimeter of the printed circuit board. As such, the result is a single laminated printed circuit board unit 150 including a printed circuit board 155 and a lead frame 160.
This method also has a number of drawbacks. For example, this method is time-consuming in that the printed circuit board's manufacturing process requires an added step--insertion of the lead frame fingers into the printed circuit board. The manufacturing process is also lengthened because additional precautionary measures must be taken to prevent damage to the lead frame fingers during manufacture of the printed circuit board.
Multi-chip modules employing a laminated printed circuit board wherein lead frame fingers become part of the printed circuit board during lamination are also known in the art.